git-new-workdir: Improve command line arg handling

The case where git-new-workdir was called without any arguments
resulted in the error message:

Not a git repository: ""

Since get_orig_gitdir was called prior to checking the number of
command line arguments.  This patch moves argument count
checking to immediately after command line parsing to ensure we
output the usage in situations where we have too few or too many
arguments.
Merge pull request #4 from aeruder/master

git-new-workdir: Improve command line arg handling